Irrlicht Engine

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// Create our blank image.
// texture_size is a dimension2du that is storing the size of our texture.
image = driver->createImage(video::ECF_A8R8G8B8, texture_size);

// Fill the image with black pixels with an alpha of 0.
// FreeType returns just an alpha channel, so we will overwrite the alpha values
// in the code below.
image->fill(video::SColor(0, 255, 255, 255));

// In Irrlicht, pitch is the number of bits for 1 row of pixels.
// We divide by u32 to return how many pixels are in one row.
// Sometimes, an image may have some unused border pixels, so the width will not
// be the same as the pitch, so always use pitch when calculating our
// location manually!
const u32 image_pitch = image->getPitch() / sizeof(u32);

// Here is where we lock the texture.  Our image is A8R8G8B8,
// so one full pixel is 32-bits.
// The first 8 are alpha, next 8 are red, then green, and the last 8 are blue.
// I cast the pointer to u32 so when I increment the pointer,
// I move forward one full pixel.
u32* data = (u32*)image->lock();

// This is a pointer to the FreeType bitmap.
u8* row = glyph->bitmap.buffer;

// Now, I iterate through the FreeType bitmap.
for (s32 y = 0; y < bits.rows; ++y)
{
    // I create a new pointer to point to the current row in the FreeType bitmap.
    u8* bitsdata = row;

    // Now, I iterate through each pixel in the FreeType bitmap row.
    // y * image_pitch + x will return the current pixel in our
    // image to manipulate.
    // *bitsdata++ will return the current alpha value I am looking at
    // in the FreeType bitmap,
    // then increment to the next value.
    // Since *bitsdata is returning an alpha value, we shift it 24 bits to the left.
    // That places at the location of alpha in a 32-bit A8R8G8B8 pixel.
    for (s32 x = 0; x < bits.width; ++x)
        data[y * image_pitch + x] |= ((u32)(*bitsdata++) << 24);

    // We want to go to the next row now,
    // so we increment our row pointer by the pitch.
    row += bits.pitch;
}
image->unlock();